Use of thermoplastic composing polyethlene glycol as additive

ABSTRACT

The present invention discloses the use in rotomolding or slush molding applications of a composition comprising a polyolefin, a processing aid and optionally a UV-stabilizer.

FIELD OF THE INVENTION

The present invention relates to the use in rotomolding or in slushmolding applications of a polyolefin composition comprising a processingaid and optionally a UV-stabilizer. The polyolefin composition can alsobe used for the production of articles by other processes such asinjection molding, cast film, blown film, calendering, sheet extrusion.

BACKGROUND OF THE INVENTION

The present invention primarily concerns the fabrication of articles byrotomolding, also called rotational molding. In rotomolding apremeasured amount of polymer is placed in one half of the mold, themold is closed and then heated until the polymer is molten. The mold isrotated so as to get good distribution of the polymer in the mold. Themold can be rotated either uniaxially or biaxially, but biaxial rotationis widely preferred, i.e. simultaneous rotation around two perpendicularaxes. In the following the mold is cooled, opened and the formed articleis removed from the mold. Rotomolding can also be used for multilayermolding, for example by using more than one polymer sequentially.Rotomolding allows the production of hollow articles with good wallthickness distribution and good mechanical properties.

Slush molding is a process that is closely related to rotomolding. Inthe following the term rotomolding is therefore used to imply both,rotomolding and slush molding applications.

The most widely used polymer in rotomolding is polyethylene. Therefore alot of effort has been invested to improve the processability ofpolyethylene in rotomolding.

U.S. Pat. No. 6,362,270 discloses polymer compositions particularlysuited for rotomolding. These polymer compositions comprise of at least94% by weight of one or more thermoplastic polymers and a maximum of 6%by weight of one or more processing additives. The thermoplastic polymermay be selected from copolymers of ethylene and styrene, ethylene and/orC₃-C₂₀ alpha-olefin homo- or copolymers, nylon, polyethyleneterephthalate, polycarbonate, acrylic polymer, polystyrene, and blendsof these polymers. Suitable processing additives include aromatic oraliphatic hydrocarbon oils, esters, amides, alcohols, acids, and theirorganic or inorganic salts as well as silicone oils, polyether polyols,glycerol monostearate (GMS), pentaerytritol monooleate, erucamide,stearamides, adipic acid, sebacic acid, styrene-alpha-methyl-styrene,calcium stearate, zinc stearate, phthalates and blends thereof. Theprocessing additive preferably decreases the composition's meltviscosity and/or elasticity at zero or low shear rates to allow for areduction in sintering time, cycle time and/or maximum mold temperature.

A recent report (L. T. Pick, E. Harkin-Jones, Third Polymer ProcessingSymposium, 28-29 Jan. 2004, Belfast, p. 259-268) shows a correlationbetween the number of bubbles in a rotomolded article and its impactperformance, with a higher number of bubbles resulting in lower impactperformance. In addition, a high number of bubbles has a negativeinfluence on the optical properties of the finished articles.

There is thus a need to provide a rotomolded article with a reducednumber of bubbles.

There is also a need to provide a rotomolded article with improvedoptical properties.

There is also a need to provide a rotomolded article with improvedmechanical properties.

It is an object of the present invention to provide a rotomolded articlebased on a polyolefin composition with improved processability inrotomolding applications.

It is another object of the present invention to provide a rotomoldedarticle based on a polyolefin composition that improves the sinteringand densification processes in rotomolding applications.

It is another object of the present invention to provide a rotomoldedarticle based on a polyolefin composition that reduces bubble formationin the rotomolding process.

It is another object of the present invention to provide a rotomoldedarticle based on a polyolefin composition that improves opticalproperties of the articles produced by rotomolding applications.

It is another object of the present invention to provide a rotomoldedarticle based on a polyolefin composition that improves mechanicalproperties of the articles produced by rotomolding applications.

It is another object of the present invention to provide a rotomoldedarticle based on a polyolefin composition that allows reducing cycletime in a rotomolding process.

It is another object of the present invention to provide a process forproducing by rotomolding an article with improved properties.

It is yet another object of the present invention to provide arotomolding process with improved densification and/or sinteringprocesses.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides rotomolded or slush moldedarticles prepared from a polyolefin composition essentially consistingof

-   -   (a) from 99% by weight to 99.999% by weight of        -   (i) a polyolefin or        -   (ii) a polyolefin composition comprising from 50% by weight            to 99% by weight of a first polyolefin and from 1% by weight            to 50% by weight of a different polymer,    -   (b) from 0.001% by weight to 1% by weight of a densification        aid;    -   (c) optionally from 0.025% by weight to 0.500% by weight of one        or more UV-stabilizers.

The present invention also discloses the use of that same composition inrotomolding and slush molding.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the temperature of air inside a mold expressed in degreescentigrade as a function of time expressed in minutes for a completecycle in rotomolding applications.

FIG. 2 shows a camera set-up used to study sintering and densificationbehavior.

FIG. 3 represents a sequence of photographs showing the coalescenceprocess.

FIG. 4 represents a series of photographs showing the bubble removalprocess.

DETAILED DESCRIPTION OF THE INVENTION

The densification aid comprises a polyethylene glycol, optionallyconsisting essentially of a mixture of a polyethylene glycol as majorcomponent with a minor component selected from the group consisting ofpolyether-block co-polyamide, thermoplastic polyurethane,polyetherester, and fluoropolymer.

By major component it is meant that such a component makes up more than50% by weight. By minor component it is meant that such a componentmakes up less than 50% by weight.

Polyether-block co-polyamides are represented by the general formulaHO—[C(O)-PA-C(O)—O-PEth-O]_(n)—H  (I)wherein PA represents the polyamide segment and PEth the polyethersegment. For example the polyamide segment can be a PA 6, PA 66, PA 11or a PA 12. The polyether segment can for example be a polyethyleneglycol (PEG) or a polypropylene glycol (PPG) or apolytetramethylenglycol (PTMG). The molecular weight M_(n) of thepolyamide sequence is usually between 300 and 15,000. The molecularweight M_(n) of the polyether sequence is usually between 100 and 6000.Such materials are commercially available for example from Atofina underthe Pebax® trade name.

The copolymers having polyamide blocks and polyether blocks aregenerally obtained from the polycondensation of polyamide blocks havingreactive end groups with polyether blocks having reactive end groups,such as, inter alia:

-   -   1) polyamide blocks having diamine chain ends with        polyoxyalkylene blocks having dicarboxylic chain ends;    -   2) polyamide blocks having dicarboxylic chain ends with        polyoxyalkylene blocks having diamine chain ends, obtained by        cyanoethylation and hydrogenation of aliphatic dihydroxylated        α,ω-polyoxyalkylene blocks called polyetherdiols; and    -   3) polyamide blocks having dicarboxylic chain ends with        polyetherdiols, the products obtained being, in this particular        case, polyetheresteramides.

The polyamide blocks having dicarboxylic chain ends derive, for example,from the condensation of polyamide precursors in the presence of achain-stopping carboxylic diacid.

The polyamide blocks having diamine chain ends derive, for example, fromthe condensation of polyamide precursors in the presence of achain-stopping diamine.

The polymers having polyamide blocks and polyether blocks may alsoinclude randomly distributed units. These polymers may be prepared bythe simultaneous reaction of the polyether and of the precursors of thepolyamide blocks.

For example, a polyetherdiol, polyamide precursors and a chain-stoppingdiacid may be made to react together. A polymer is obtained whichessentially has polyether blocks and polyamide blocks of very variablelength, but in addition the various reactants that have reactedrandomly, which are distributed in a random fashion along the polymerchain.

A polyether diamine, polyamide precursors and a chain-stopping diacidmay also be made to react together. A polymer is obtained which hasessentially polyether blocks and polyamide blocks of very variablelength, but also the various reactants that have reacted randomly, whichare distributed in a random fashion along the polymer chain.

The amount of polyether blocks in these copolymers having polyamideblocks and polyether blocks is advantageously from 10 to 70% andpreferably from 35 to 60% by weight of the copolymer.

The polyetherdiol blocks may either be used as such and copolycondensedwith polyamide blocks having carboxylic end groups, or they may beaminated in order to be converted into polyetherdiamines and condensedwith polyamide blocks having carboxylic end groups. They may also beblended with polyamide precursors and a diacid chain stopper in order tomake the polymers having polyamide blocks and polyether blocks withrandomly distributed units.

The number-average molar mass M_(n) of the polyamide blocks is usuallybetween 300 and 15,000, except in the case of the polyamide blocks ofthe second type. The mass M_(n) of the polyether blocks is usuallybetween 100 and 6000.

As regards the polyetheresters, these are copolymers having polyesterblocks and polyether blocks. They generally consist of soft polyetherblocks, which are the residues of polyetherdiols, and of hard segments(polyester blocks), which usually result from the reaction of at leastone dicarboxylic acid with at least one chain-extending short diol unit.The polyester blocks and the polyether blocks are generally linked byester linkages resulting from the reaction of the acid functional groupsof the acid with the OH functional groups of the polyetherdiol. Theshort chain-extending diol may be chosen from the group consisting ofneopentyl glycol, cyclohexanedimethanol and aliphatic glycols of formulaHO(CH₂)_(n)OH in which n is an integer varying from 2 to 10.Advantageously, the diacids are aromatic dicarboxylic acids having from8 to 14 carbon atoms. Up to 50 mol % of the dicarboxylic aromatic acidmay be replaced with at least one other dicarboxylic aromatic acidhaving from 8 to 14 carbon atoms, and/or up to 20 mol % may be replacedwith a dicarboxylic aliphatic acid having from 2 to 12 carbon atoms.

As examples of dicarboxylic aromatic acids, mention may be made ofterephthalic, isophthalic, dibenzoic, naphthalenedicarboxylic acids,4,4′-diphenylenedicarboxylic acid, bis(p-carboxyphenyl)methane acid,ethylenebis(p-benzoic acid), 1,4-tetramethylenebis(p-oxybenzoic acid),ethylenebis(paraoxybenzoic acid) and 1,3-trimethylene bis(p-oxybenzoicacid). As examples of glycols, mention may be made of ethylene glycol,1,3-trimethylene glycol, 1,4-tetramethylene glycol, 1,6-hexamethyleneglycol, 1,3-propylene glycol, 1,8-octamethylene glycol,1,10-decamethylene glycol and 1,4-cyclohexylenedimethanol. Thecopolymers having polyester blocks and polyether blocks are, forexample, copolymers having polyether blocks derived from polyetherdiols, such as polyethylene glycol (PEG), polypropylene glycol (PPG) orpolytetramethylene glycol (PTMG), dicarboxylic acid units, such asterephthalic acid, and glycol (ethanediol) or 1,4-butanediol units. Thechain-linking of the polyethers and diacids forms soft segments whilethe chain-linking of the glycol or the butanediol with the diacids formsthe hard segments of the copolyetherester. Such copolyetheresters aredisclosed in patents EP 402 883 and EP 405227. These polyetheresters arethermoplastic elastomers. They may contain plasticizers.

Polyetheresters can for example be obtained from Du Pont Company underthe Hytrel® trademark.

As regards the polyurethanes, these in general consist of soft polyetherblocks, which usually are residues of polyetherdiols, and hard blocks(polyurethanes), which may result from the reaction of at least onediisocyanate with at least one short diol. The short chain-extendingdiol may be chosen from the glycols mentioned above in the descriptionof the polyether esters. The polyurethane blocks and polyether blocksare linked by linkages resulting from the reaction of the isocyanatefunctional groups with the OH functional groups of the polyether diol.

Thermoplastic polyurethanes can for example be obtained from ElastogranGmbH under the Elastollan® trade name or from Dow Chemical Company underthe Pellethane® trade name.

Polyethylene glycols have the general formulaH—(OCH₂CH₂—)_(n)OH  (I)

Polyethylene glycols are commercially available in a wide range ofmolecular weights and viscosities. Depending upon their molecularweights polyethylene glycols can be liquid or solid. The polyethyleneglycols used in the present invention usually have an average molecularweight from 100 to 2000 g/mol and more preferably from 150 to 700 g/mol.Suitable polyethylene glycols can for example be obtained from DowChemical Company or BASF under the Carbowax® and Plurol E® trade names.

The fluoropolymers suited as processing aid in the present invention arefor example polymers of vinylidene fluoride (H₂C═CF₂) and/or copolymersof vinylidene fluoride and hexafluoropropylene (F₂C═CF—CF₃). Though thecopolymers of vinylidene fluoride and hexafluoropropylene do not haveelastomeric properties they are commonly referred to as“fluoroelastomers”. The content of the comonomer hexafluoropropylene ina fluoroelastomer is usually in the range of 30 to 40% by weight.Fluoropolymers suited as processing aids in the current invention arefor example commercially available under the Dynamar®, Viton® and Kynar®trade names from Dyneon, DuPont-Dow Elastomers or Atofina.

Polyethylenes prepared with a Ziegler-Natta or with metallocene catalystor with late transition metal catalyst systems are typically used inrotomolding applications. Other materials can also be used, such as forexample polypropylene. Linear low density polyethylene is preferablyused as disclosed for example in “Some new results on rotational moldingof metallocene polyethylenes” by D. Annechini, E. Takacs and J.Vlachopoulos in ANTEC, vol. 1, 2001.

The preferred polyolefin used in the composition according to thepresent invention is a homo- or co-polymer of ethylene produced with acatalyst comprising a metallocene on a silica/aluminoxane support. Morepreferably, the metallocene component is ethylene-bis-tetrahydroindenylzirconium dichloride or bis-(n-butyl-clopentadienyl) zirconiumdichloride ordichloro-(dimethylsilylene)bis(2-methyl-4-phenyl-indenylidene) zirconiumdichloride. The most preferred metallocene component isethylene-bis-tetrahydroindenyl zirconium dichloride.

In this description, the term copolymer refers to the polymerizationproduct of one monomer and one or more comonomers. Preferably themonomer and the one or more comonomers are alpha-olefins with two to tencarbon atoms, with monomer and comonomer(s) being differentalpha-olefins. More preferably the monomer is either ethylene orpropylene and the one or more comonomers are alpha-olefins with two toeight carbon atoms. Most preferably the monomer is ethylene and thecomonomer is either 1-butene or 1-hexene.

The melt index of the polyethylene or polypropylene resin preferablyused in the present invention typically falls in the following ranges:

-   -   If the polyolefin of the present invention is a homo- or        co-polymer of ethylene, Its melt index MI2 is typically in the        range 0.1 to 25 dg/min, preferably in the range 0.5 to 15 dg/min        and most preferably in the range 1.5 to 10 dg/min. The MI2 is        measured according to ASTM D 1283 at a temperature of 190° C.        and a load of 2.16 kg.    -   If the polyolefin of the present invention is a homo- or        copolymer of propylene, its melt flow index (MFI) is typically        in the range 0.1 to 40 dg/min, preferably in the range 0.5 to 30        dg/min and most preferably in the range 1 to 25 dg/min. The MFI        is measured according to ASTM D 1283 at a temperature of 230° C.        and a load of 2.16 kg.

For the homo- and co-polymers of ethylene that can be used in thepresent invention, the density is typically in the range 0.910 to 0.975g/ml and preferably in the range 0.915 to 0.955 g/ml, and mostpreferably in the range 0.925 to 0.945 g/ml. The density is measuredaccording to ASTM D 1505 at 23° C.

The polyolefins of the present invention may also have a bi- ormultimodal molecular weight distribution, i.e. they may be a blend oftwo or more polyolefins with different molecular weight distributions,which can be blended either physically or chemically, i.e. producedsequentially in two or more reactors.

The polydispersity D of the polyolefins used in the present invention isdefined as the ratio Mw/Mn of the weight average molecular weight Mwover the number average molecular weight Mn. It is in the range 2 to 20,preferably 2 to 8, more preferably less than or equal to 5, and mostpreferably less than or equal to 4, the latter range being typicallyassociated with the preferred metallocene-prepared polyethylene resins.

The polyolefins of the present invention may also comprise otheradditives such as for example antioxidants, acid scavengers, antistaticadditives, fillers, slip additives or anti-blocking additives.

When a polyolefin composition is used as starting material, thecomposition comprises:

-   -   from 50 to 99% by weight of a first polyolefin, preferably        polyethylene;    -   from 1 to 50% by weight of a second polymer, which is different        from the processing aid, and which is preferably selected from        the group consisting of polyamide, copolyamide, a second        polyolefin different from the first one, copolymers of ethylene        and vinyl acetate (EVA), copolymers of ethylene and vinyl        alcohol (EVOH), polystyrene, polycarbonate and polyvinyl        chloride (PVC).

It is also possible to use a polyolefin comprising a densification aidas one or more layers of a multilayered rotomolded article with theother layers comprising a polymer preferably selected from the groupconsisting of polyamide, copolyamide, a second polyolefin different fromthe first one, copolymers of ethylene and vinyl acetate (EVA),copolymers of ethylene and vinyl alcohol (EVOH), polystyrene,polycarbonate and polyvinyl chloride (PVC).

The one or more UV-stabilizers can be selected from any knownUV-stabilizer known in the art. The preferred UV-stabilizers arehindered amine light stabilizers (HALS). Commercially available examplesof HALS include Chimassorb® 944, Tinuvin® 622 or Tinuvin® 783 from CibaSpecialty Chemicals.

Surprisingly, it has been found that the addition of 0.001% by weight to1% by weight of a processing aid improves the processability of apolyolefin in rotomolding by modifying the sintering and thedensification behavior.

The use of a processing aid according to the present invention resultsin cycle time reductions of at least 10%, preferably by at least 20%. Inorder to obtain the same number of bubbles in the rotomolded articlesthe peak internal air temperature (PIAT) can be reduced by at least 10degrees centigrade.

Even more surprisingly, it has been found that the further addition offrom 0.025% by weight to 0.500% by weight of one or more UV-stabilizersto the composition described hereabove comprising 0.001% by weight to 1%by weight of a processing aid even further improves the processabilityof the polyolefin in rotomolding.

It is believed that there is a synergy between the processing aid andthe UV-stabilizer, and it is thus preferred to use both.

In rotomolding a premeasured amount of polymer is placed in one half ofthe mold, the mold is dosed and then heated until the polymer is molten.The mold is rotated so as to get an even distribution of the polymer inthe mold. The mold can be rotated either uniaxially or biaxially, butbiaxial rotation is widely preferred, i.e. simultaneous rotation aroundtwo perpendicular axes. In the following step the mold is cooled, openedand the formed article is removed from the mold.

The rotomolding cycle comprises three main steps, each of which has animpact on cycle time and the properties of the so-produced article. Thethree steps comprise:

-   -   sintering or coalescence,    -   densification or bubble removal, and    -   crystallization.

This is Illustrated in FIG. 1 giving the air temperature in the mold,expressed in degrees centigrade, as a function of time, expressed inminutes, during an exemplary molding cycle. The first inflexion in thecurve noted as point A marks the beginning of the sintering orcoalescence of the polymer particles. Sintering in the presentapplication represents the coalescence of the polymer particles. Thenext inflexion in the curve noted as point B marks the beginning of thedensification process of the molten polymer. Densification in thepresent application means bubble removal. Throughout this applicationsintering and densification are seen as two distinct processes; theyvary independently with the rotomolding parameters and with the resinproperties.

Point C on the curve represents the Peak Internal Air Temperature(PIAT), followed by point D that marks the beginning of thecrystallization process. Point E is associated with the time at whichthe rotomolded article is completely solidified and starts receding fromthe walls of the mold. Point F marks the opening of the mold, i.e. theend of the rotomolding cycle.

The present invention is mostly concerned with the modification of thepolymer behavior in the sintering (coalescence) and densification(bubble removal) phases of the rotomolding cycle and slush moldingcycle. Sintering is measured according to a method described for exampleby Bellehumeur et al. (C. T. Bellehumeur, M. K. Bisaria, J.Vlachopoulos, Polymer Engineering and Science, 36, 2198, 1996).Densification and bubble formation has been discussed by Kontopoulo etal. (M. Kontopoulo, E. Takacs, J. Vlachopoulos, Rotation, 28, Jan.2000). During melting air pockets or bubbles are trapped, thus delayingthe formation of a homogeneous melt and also affecting the aestheticaland/or mechanical properties of the finished product.

For the present invention a charge-coupled device (CCD) camera was usedto characterize the properties of polyolefin powders during arotomolding cycle or during sintering and/or densification simulations.

EXAMPLES

Characterization of the processing behavior was analyzed using amegapixel progressive scan interline CCD with on-chip circuitscommercially available from Kodak. It has the following characteristics:

-   -   architecture: interline CCD, progressive scan, non-interlaced    -   pixel count: 1000 (H)×1000 (V)    -   pixel size: 7.4 μm(H)×7.4 μm(V)    -   photosensitive area: 7.4 mm(H)×7.4 mm(V)    -   output sensitivity: 12 μV/electron    -   saturation signal 40,000 electrons    -   dark noise: 40 electrons rms    -   dark current (typical): <0.5 nA/cm²    -   dynamic range: 60 dB    -   quantum efficiency at 500, 540, 600 nm 36%, 33%, 26%    -   blooming suspension: 100×    -   image lag: <10 electrons    -   smear: <0.03%    -   maximum data rate: 40 MHz/channel (2 channels)    -   integrated vertical dock drivers    -   integrated correlated double sampling (CDS)    -   integrated electronic shutter driver

The high performance 15-bit (16 bits minus 1 bit for control) CCD sensorwith transparent gate electrode provides 32,768 unsigned levels of gray,allows the acquisition of about 10,000 frames/s and covers a broadspectrum of from 400 to 1000 nm.

The camera set-up used to study sintering and densification behavior isillustrated in FIG. 2 with the CCD camera (1), the IR probe (2), thecomputer (3), the heating system (4) and the annular lighting system(5). A typical example for sintering is shown in FIG. 3 and a typicalexample for densification or bubble removal in FIG. 4.

The progressive disappearance of bubbles as a function of time andtemperature is clearly and instantaneously followed. In addition to thevisual aspect, the computer instantaneously produces a set of parametersresulting from picture analysis. These parameters are explained in TableI.

TABLE I Parameter Unit Description Ex — picture number t min time ofpicture T ° C. IR temperature of sample N — number of bubbles on thepicture Na mm⁻² number of bubbles per mm² A μm² total area covered bybubbles Aa — percentage of total picture area covered by bubbles D_(av)μm average distance between 2 bubbles S μm² average area of one bubbleCr μm perimeter of one bubble based on Crofton's integral D_(eq) μmequivalent diameter of one bubble L μm largest side of one bubble W μmsmallest side of one bubble LO degree orientation of the largest side WOdegree orientation of the smallest side

The average distance between two bubbles D_(av) is defined asD _(av)=4(1−Aa)/Svwherein Sv=4π(D_(eq)/2)²·Aa/((4π/3) (D_(eq)/2)³)wherein the equivalent bubble diameter D_(eq) is defined in terms of theaverage surface of one bubble S by the equation S=4π(D_(eq)/2)².

The base polyethylenes were supplied as pellets. The pellets were groundat 40 to 80° C. on commercial grinding equipment, e.g. a Wedco Series SEmachine, to a powder with grain sizes from 100 μm to 800 μm. Theprocessing aid or a blend of processing aids and a UV-stabilizer or ablend of UV-stabilizers were added to the powder in commercial mixingequipment.

Irganox® B 215 is a blend of Irgafos® 168 and Irganox® 1010 and iscommercially available from Ciba Specialty Chemicals. Tinuvin® 783 is aUV-stabilizer commercially available from Ciba Specialty Chemicals.Cyasorb THT® 4611 and Cyasorb THT® 4802 are UV-stabilizers commerciallyavailable from Cytec Industries. Carbowax® E300 can be obtained from DowChemical Corporation.

Examples 1 to 4 and Comparative Example 1

The polyethylene used for examples 1 to 4 and comparative example 1 wasa monomodal polyethylene with a MI2 of 8.0 dg/min and a density of 0.934g/ml; it was produced using a supported metallocene catalyst system. Itis commercially available from Atofina under the name Finacene® M3582.

The processing aids, UV-stabilizers and other additives are given inTable II, together with their respective amounts.

The samples were evaluated on a 10 L canister prepared by rotomoldingusing a commercial rotomolding equipment. Peak Internal Air Temperature(PIAT) was 210° C. in all cases.

TABLE II Comp. Ex. 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 MI2 8.0 8.0 8.0 8.0 8.0(dg/min) Density 0.934 0.934 0.934 0.934 0.934 (g/ml) Irganox ® 15001500 1500 1500 1500 B 215 (ppm) Zinc stearate 1500 (ppm) Carbowax ® 5001000 500 500 E300 (ppm) Tinuvin ® 1500 783 (ppm) Cyasorb 1000 THT ® 4611(ppm)

Using the polymer compositions of examples 1 to 4 rotomolded articleswith a low number of bubbles could be obtained.

1. A method for the preparation of a molded article formed of an olefinpolymer comprising: a) providing a molding feedstock comprising aparticulate α-olefin polymer composition, wherein the particulate olefinpolymer is selected from ethylene homopolymers, propylene homopolymers,or copolymers of ethylene or propylene and C₂-C₈ olefin polymers; b)providing a densification aid comprising polyethylene glycol, c)introducing said olefin polymer composition and said densification aidin an amount of less than 1 weight percent based upon the amount of saidolefin polymer into a mold cavity configured to provide a mold of adesired shape; d) heating said olefin polymer composition in thepresence of said densification aid in said mold cavity to a temperaturesufficient to provide a molten state to form a molded article configuredto the shape of said mold cavity, wherein the mold is rotated during theheating of said olefin polymer to a molten state; e) thereafter coolingsaid molten polymer composition within the confines of said mold cavityto a temperature effective to solidify said olefin polymer compositionwithin the confines of said mold cavity; and f) retrieving saidsolidified molded article from said mold cavity, wherein subsequent tothe introduction of said olefin polymer composition and saiddensification additive into said mold cavity, said polymer is heated toa sintering temperature in which individual particles of said olefinpolymer are coaleased followed densification of said molten polymer andthe increase of the temperature in said mold cavity to a peak internalair temperature within said mold cavity followed by the cooling of saidmold cavity to a crystallization temperature for the solidification ofsaid molten polymer to produce said molded article wherein the peakinternal air temperature within said mold cavity to produce a designatednumber of air bubbles within said molten polymer is reduced by anincrement of at least 10° C. from the peak internal air temperaturerequired to produce the designated number of air bubbles for a polymermolded under identical conditions without the introduction of saiddensification aid into said mold cavity.
 2. The method of claim 1wherein said densification aid comprises said polyethylene glycol as amajor component and in addition comprises a minor component selectedfrom the group consisting of a fluoropolymer, a thermoplasticpolyurethane, a polyetherester and a polyether-block copolymide.
 3. Themethod of claim 1 wherein said polyethylene glycol has an averagemolecular weight with the range of 100-2,000.
 4. The method of claim 1wherein said polyethylene glycol has an average molecular weight withthe range of 150-700.
 5. The method of claim 1 further comprisingintroducing at least one ultraviolet (UV) stabilizer into said moldcavity in an amount of no more than 0.5 weight percent based upon theamount of olefin polymer supplied to said mold cavity.
 6. The method ofclaim 1 wherein said olefin polymer comprises an ethylene or propylenehomopolymer or copolymer prepared by the polymerization of ethylene orpropylene in the presence of a metallocene based polymerization system.7. The method of claim 2 wherein the minor component of saiddensification aid comprises a thermoplastic polyurethane in an amountless than the amount of said polyethylene glycol.
 8. The method of claim2 wherein said minor component of said densification aid comprises apolyetherester in an amount less than the amount of said polyethyleneglycol.
 9. The method of claim 2 wherein said minor component of saiddensification aid comprises a polyether block copolyamide in an amountless than the amount of said polyethylene glycol.
 10. The method ofclaim 1 further comprising introducing a UV stabilizer into said moldcavity concomitantly with the introduction of said polymer compositionand said densification aid.
 11. The method of claim 10 wherein said UVstabilizer is introduced into said mold cavity in an amount within therange of 0.025-0.5 weight percent based upon the amount of said olefinpolymer.
 12. The method of claim 10 wherein said UV stabilizer is ahindered amine light stabilizer.
 13. The method of claim 1 wherein saidolefin polymer composition comprises a first olefin polymer and a secondpolymer which is different from said processing aid and said firstolefin polymer which is present in an amount less than said first olefinpolymer and said second polymer is selected from the group consisting ofa polyamide, a copolyamide, a polyolefin, a copolymer of ethylene andvinyl acetate or vinyl alcohol, polystyrene, polycarbonate, andpolyvinyl chloride.
 14. A method for the preparation of a molded articleformed of an olefin polymer comprising: a) providing a particulatemolding feedstock comprising a mixture of an α-olefin polymercomposition, wherein the olefin polymer composition is selected fromethylene homopolymers, propylene homopolymers or copolymers of ethyleneor propylene and C₂-C₈ olefin polymers and a densification aidcomprising a polyethylene glycol in an amount of less than 1 weightpercent based upon the amount of said olefin polymer composition; b)introducing said molding feedstock into the mold cavity of a rotationalmode; c) rotating said mold about at least one axis; d) during therotation of said mold, heating said olefin polymer compositionincorporating said densification aid to a temperature sufficient toprovide a molten state of said polymer composition to form a moldedarticle configured to the shape of said mold cavity; e) thereaftercooling said molten polymer composition within the confines of said moldcavity to a temperature effective to solidify said olefin polymercomposition within the confines of said mold cavity; and f) retrievingsaid solidified molded article from said mold cavity; wherein subsequentto the introduction of said olefin polymer composition and saiddensification additive into said mold cavity, said polymer is heated toa sintering temperature in which individual particles of said olefinpolymer are coaleased followed densification of said molten polymer andthe increase of the temperature in said mold cavity to a peak internalair temperature within said mold cavity followed by the cooling of saidmold cavity to a crystallization temperature for the solidification ofsaid molten polymer to produce said molded article wherein the peakinternal air temperature within said mold cavity to produce a designatednumber of air bubbles within said molten polymer is reduced by anincrement of at least 10° C. from the peak internal air temperaturerequired to produce the designated number of air bubbles for a polymermolded under identical conditions without the introduction of saiddensification aid into said mold cavity.
 15. The method of claim 14wherein said densification aid comprises said polyethylene glycol as amajor component and in addition comprises a minor component selectedfrom the group consisting of a thermoplastic polyurethane, apolyetherester, a polyethylene block copolyamide, and a fluoropolymer.